If you see a crescent moon, you can tell waxing from waning by which side is illuminated and the direction it’s growing or shrinking. How to tell using illumination
- In the Northern Hemisphere:
- Waxing crescent: the right side is lit and the crescent curves toward the left. The illuminated portion is increasing toward the full moon.
- Waning crescent: the left side is lit and the crescent curves toward the right. The illuminated portion is decreasing toward the new moon.
- In the Southern Hemisphere, the signs are reversed: the left side is lit for waxing and the right side for waning, with the same “growing toward full” versus “shrinking toward new” idea.
Alternate quick tips
- If the crescent appears shortly after sunset and grows taller over successive evenings (growing toward a first quarter and then full moon), it’s waxing.
- If the crescent appears in the early morning hours and appears to shrink toward the next new moon, it’s waning.
- For a crescent near the horizon after sunset, remember the mnemonic that waxing crescents typically appear on the right-hand side in the Northern Hemisphere, while waning crescents appear on the left-hand side.
Notes
- The key is always which side is illuminated and whether the visible illuminated portion is increasing or decreasing over successive nights.
- The Moon’s phase direction (waxing vs waning) is the same globally; only the side correspondence to waxing/waning changes with hemisphere.
